Vented roofs work in all climate zones, but depend on a few critical details: 1) Both soffit and ridge vents in each rafter bay. Balanced openings work, but it’s better if the soffit inlets are larger than the outlet at the ridge to positively pressurize the vent space. 2) A perfect air barrier, either at the ceiling or at the baffle. Avoid can lights, but if you must use them, make the baffle the air barrier. 3) A continuous baffle to prevent ventilation air from washing over and passing through fibrous cavity insulation. The challenge with a vented cathedral ceiling assembly is getting enough insulation into the limited space of a rafter cavity to meet energy code minimums. Consider either using deeper rafters (using I-joists or parallel-chord trusses) or adding a layer of rigid foam to the underside of the rafters and strapping the ceiling for drywall. The rigid-foam option has the advantage of providing a thermal break.

Questions about building cathedral ceilings have troubled energy-conscious builders for decades. Common questions include: With only the depth of the rafter to work with, how do we get enough insulation R-value to meet the energy code, much less meet high-performance building standards? How do we even come close to having room for ventilation? If we leave out the roof ventilation to maximize insulation depth, how can we avoid ice dams? How do we prevent condensation from "raining" down from skylights and recessed lights?There are answers to all these questions, which is fortunate because the risks of water damage and a...